The present invention is applicable to a broad class of radiation detectors in which the energy of a detected particle (typically, an x-ray photon, in the context described below) is inferred from the charge collected during the duration of a detector pulse. The number of electrons produced in a photomultiplier that detects scintillation emission in an x-ray scintillation detector is but one example relating to the class of detectors to which the present invention may be advantageously applied. In this class of detectors, the area under a plot of the pulse amplitude as a function of time is used to determine the energy of the detected particle. In such radiation detectors, it is important that the pulses be treated separately and distinctly, for purposes of inferring the integrated area under the respective pulses. If subsequent pulses pile up on top of the tails of preceding pulses, the residual amplitude of the preceding pulse tails will be imputed to the integrated areas of successive pulses.
Therefore, it has long been the practice, in the application of energy-resolving detectors, to “shape” detector pulses by shortening the pulse tails while preserving the integrated area of the pulse. This is typically accomplished by operation of a pulse processor, which will typically include a digital signal processor (DSP) executing stored software instructions. The output of the pulse processor is influenced by the values of various pulse processing parameters employed for calculation, the values of which may be prestored in the memory of or associated with the DSP. One such pulse processing parameter is the pulse shaping time. A discussion of the selection of pulse shaping time and other pulse processing parameters is set forth in Knoll, Radiation and Detection, 3rd Edition, John Wiley and Sons (2000), the disclosure of which is incorporated herein by reference.
The energy spectrum of x-rays fluoresced from a sample has been used for decades to determine its elemental and chemical composition. Applications for x-ray fluorescence (XRF) techniques are extremely wide-ranging, and include, for example, sorting alloys, analyzing soil, determining the lead concentration in painted walls, measuring quantities of toxic elements in consumer goods, and determining the thickness and composition of electroplatings. Hand-held XRF instruments, such as the Thermo Scientific Niton XRF instruments, are often purchased for multiple uses. Yet, for each specific application there is generally an optimum x-ray energy spectrum that most effectively fluoresces the sample. It is standard practice to create the optimum spectrum by changing maximum energy and filtration of the excitation beam that is incident on a sample. The source of x-ray emission may be an x-ray tube, or other x-ray source such as a radioactive source. Metal alloys, for example, are analyzed with quite different x-ray spectrum parameters than are used for studying soil. And a single test of an alloy or a soil may involve consecutive, pre-programmed changes of the high voltage and/or the filtration so as to most effectively analyze a wide range of elements in the sample. It is also standard practice to automatically adjust the intensity of the x-ray beam to maximize the number of x-rays collected during a given test time.
Co-pending U.S. patent application Ser. No. 12/426,022, to Dugas, entitled “Automated X-Ray Fluorescence Analysis” (the Application of Dugas), the entire disclosure of which is incorporated herein by reference, describes how the selection of the optimum shape of the x-ray energy spectrum incident on the target can be automated so that the user does not need prior knowledge of the type of sample being measured. However, the Dugas method does not address the spectrum of x-rays detected from the target.
Co-pending U.S. patent application Ser. No. 12/142,737, to Camus et al., incorporated herein by reference, discusses implications of multiple detection events within the course of a detector shaping time. It does not address dynamic variation of shaping times in response to resolution requirements.